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8/8/2019 [SG Symposium] FuelCellUAV CSU 1
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Design and Test of a 24 Hour Fuel Cell Unmanned Aerial Vehicle
(FCUAV)
Derek Keen, Grant Rhoads, Tim Schneider, Brian Taylor, Nick Wagner
Colorado State University
Faculty Advisor: Dr. Thomas Bradley
Abstract
Long endurance unmanned aerial vehicles
(UAVs) have increasing value as a low cost,
autonomous reconnaissance and remotesensing platform for research, commercial
and military missions. Current multi-
disciplinary optimization techniques andfuel-cell technologies have the potential toincrease the endurance of such systems
significantly. Research performed by Dr.
Thomas Bradley while at Georgia Tech.University showed that significant gains
over current systems were possible. This
aircraft, powered by a polymer electrolytemembrane (PEM) fuel cell, with compressed
hydrogen storage, and integrated
conditioning systems, is an effort to verify
and continue his research. The flight testresults will be compared with the
optimization research leading to this aircraft
design and flight tests, as well as topublished results of similar 0.51kW long-
endurance unmanned aircraft. As per the
research analysis, the flight tests will verify
the increased endurance of greater than 24hrs of flight time. Further improvements to
the system and planned future work will
possibly include switching to a liquid
hydrogen storage system for greatlyincreased endurance. The practical
implications of this effort are wide reaching
and pertinent both to further research workand current UAV customers.
1 Airframe DesignThe research and aircraft demonstrator
undertaken by Dr. Thomas Bradley at Georgia
Tech University provided the set point for the
airframe that was constructed during the
summer of 2009. As noted above the goal ofthis aircraft is to demonstrate the use of a
gaseous hydrogen supplied PEM fuel cell
system. Based on Dr. Bradleys research, and a
custom designed 600 W fuel cell from United
Technologies Research Center (UTRC), we had
an optimal threshold in terms of weight, size,
and aerodynamics that had to be met in order
to achieve the predicted 24 hour flight later
on[3]. The design decisions made as a result
are discussed in the following sections.
1.1 Wing AssemblyAll of the lifting surfaces on this aircraft are
originally from the Blue Explorer 5m composite
sailplane sold by Northeast Sailplane Products
. This approach allowed for a shorter
development time, while providing a high
quality, aerodynamically efficient and stable
wing to begin the design process. To maintain
the aircraft stability, care was taken to ensure
that the center of gravity was directly beneath
the quarter chord of the wing. The quarter
chord refers to the position one quarter of the
distance between the leading and trailing
edges. The existing fastener attachment points
were used to connect the wing to the carbon
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fiber spine via custom ASTM 6061 aluminum
mounts. This carbon fiber spine is discussed in
further detail below.
The wing is a three piece spar and monocoque
composite structure, with eight internal servomotors controlling split ailerons, flaps, and
spoilers. The airfoil is a modified HQW 2.5 for
high lift at moderate speeds and low Reynolds
numbers. The lifting capacity of this wing was
determined sufficient based on the coefficient
of lift and wing area as compared with the
computational design tool [2,3] developed at
Georgia Tech as well as the published metrics of
the acceptable G-loading.
1.2 Tail AssemblyThe empennage of this aircraft was taken from
the Blue Explorer sailplane mentioned above. It
utilizes a traditional configuration with the
elevator positioned very close to the horizontal
datum plane of the main wing making an upside
down T with the rudder. Using a traditional
configuration allowed for the application of
previously developed autopilot flight controls.
The rudder and elevator are controlled by
separate servo motors located in front of the
structural hydrogen tank. These are connected
to their respective control surfaces via graphite
control rods along the carbon fiber spine. As
stated above, the empennage assembly was
taken from a pre-constructed sailplane. It is
bonded to the carbon fiber spine that extends
from the fuselage structure using wood
buttresses and epoxy. Plastic body filler was
used to ensure a premium surface finish and
smooth spine-empennage transition.
1.3 Fuselage StructureDue to the large frontal surface area of the
hydrogen storage tank, much of the fuselage
shape was dictated by this tank. Acting as a skin
between the internal components and the
environment, a thin layer of fiberglass was
manufactured to enclose all components except
the infrared sensors used by the autopilot
telemetry. Due to the shape and size of the
hydrogen storage tank, a cylindrical fuselage
shape was used with conical shapes to
transition from the nose to the tail. Using hose
clamps and custom fixtures, the hydrogen tank
is secured to a one inch diameter hollow carbon
fiber tube. This serves as the spine of the plane
providing structural support along the length
from the front motor mount all the way to the
empennage in the rear. As the main structure of
the aircraft, everything stems form the carbon
fiber spine. The wings, servo motors,
electronics, propeller motor and hydrogen
storage tank are attached to this spine via ASTM
6061 lightweight aluminum brackets that were
manufactured using a computer numeric
controlled (CNC) milling machine. All structural
components were computationally tested
against theory using finite element analysis.
1.4 Landing GearThis is the one aspect of the airplane that has
caused a number of problems during the testing
stages, though it will be replaced by a skid plate
for the final 24 hr flight. The difficulties
presented with this aircraft are its large size and
weight, and the ground clearance needed for
the large diameter propeller (20+ inches).
The initial landing gear setup was a composite
two-wheel tail-dragger configuration which,
while lightweight, was structurally unstable and
turned out to be too narrow. Following this a
mono-wheel configuration with wing skids was
employed, but proved to be too unstable for
use on a multi-flight aircraft. The landing gear
design has since moved to a traditional tricycle
configuration with two wheels of a large
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wheelbase behind the center of gravity and a
single wheel directly behind the propeller with
steering controlled by the rudder servo motor.
Designed into this configuration is a lower angle
of attack to increase the acceleration during
initial take off. Angle of attack refers to the
difference between the horizontal datum plane
and the angle made by the wing in which zero
lift is produced. While this reduces lift
temporarily, it also reduces drag significantly
allowing the plane to achieve a higher velocity
in a shorter distance. Once the desired velocity
is achieved, the elevator is moved quickly to
induce high lift for take off. This tricycle landing
gear configuration provides more stability and
control while permitting lower induced drag.
These advantages come with the minor cost of
additional weight.
2 Autopilot SystemIntegration
For the hands-free control of this aircraft and
optimal flight management we have integrated
the open source Paparrazzi autopilot developed
by Ecole Nationale de lAviation Civile in France
and used by a number of other research UAVs
(USU-OSAM, USU Aggie Air Remote Sensing,
UCSD, U of Arizona Autonomous Glider, Team
UAV UALR). This flexible ARM7 based system
uses IR (Infrared) Thermopiles for horizon
sensing on the pitch and roll axes of the aircraft.
For the flight pattern and altitude control of the
aircraft, a small uBlox LEA-5H GPS receiver is
used. With the included transceiver system,
waypoints and other
commands can be
given and
performance data
obtained from the
aircraft throughout the flight.
2.1 IR SensorsThe use of IR sensors for attitude (pitch, and
roll) control is based on the principle that the
ambient temperature IR signal from the ground
and the sky are distinctly different. While
terrain, and weather can have an impact on this
form of sensing, it is remarkably robust, and all
of our flight testing will be performed over
virtually flat terrain. Yaw control is provided
primarily by
the GPS
waypoint
commands
and any
coordinated
flight control
schemes
written in the
controller.
2.2 GPS ReceiverThe GPS Receiver is a combination of the u-Blox
chipset with Sarantels SL1206 helical antennato produce an incredibly sensitive 50 channel
GPS receiver. Some of the advantages of this
receiver is the 2 Hz update rate, low power, and
small form factor. The Sarantel
antenna also has its own filtering
giving high immunity to RF
interference.
2.3 Transceiver SystemThe transceivers used for communicating
between the ground station and the aircraft are
the Digi XBee Pro 900 RPSMA and
allow a very reliable and simple
Figure 2 - Diagram of IR sensing
Figure 1 - Autopilot board
Figure 3 - GPS receiver/ante
Figure 4 - XBee transceiv
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communication. This low power, high data rate
wireless module allows for up to 6 miles line of
site communication and have been tested to
work well with other wireless modules on the
aircraft.
2.4 Processing and Servo ControlThe processing of sensor readings and
outputting servo control is based on common
PID control. The desired closed loop dynamics
of flight are tuned by changing proportional,
integral, and derivative gains in the autopilot
software either permanently in the code or in
flight using the ground station software. The
critical core of the autopilot code has been
tested formally using Lustre.
2.5 Graphical InterfaceThe ground station interface for the autopilot
runs in a linux environment. Currently our
ground station consists of a laptop running
Ubuntu linux with the Paparazzi Center
software installed. When a flight is executed, a
satellite image of the current aircraft location
and flight plan is loaded. Here we are able to
keep track of important aspects of the plane
like battery voltage, GPS signal, altitude,
location, and autopilot mode (manual, wing
leveling, fully autonomous). The software also
records the flight for future playback.
3 Battery Power SystemThe battery power system in use is to readily
and safely provide multiple flights for flight
testing and data acquisition. This data will be
used to determine the final setup of the fuel cell
power management.
The current heavy-duty power system in the
aircraft uses 2, 5000 mAh Lithium polymer
batteries to provide power to a Hacker A60-18L
motor through a Phoenix 110 speed controller.
This setup is capable of delivering over 2kW of
power. The previously attempted flight tests
using Axi motors were thwarted by an
overloaded speed controller, shorted motor
coils, and broken magnets, thus the switch to
the more durable system despite a 1lb weight
penalty.
4 Fuel Cell System4.1 PEM Fuel CellThe 33 - cell stack we will be using is developed
specifically for this application by United
Technologies Research Center. It is a 600 W
nominal system at max power and operates at
200 W for cruise performance. Its weight is
1.68 kg, providing 357 W/kg at max power with
a hydrogen utilization of 90%. See Figure 6 for
characteristics.
4.2 Hydrogen StorageThe hydrogen is stored in a 9L, 4.5 kg composite
wound pressure vessel at 5500 Psi (MCS
International). Pressure regulation is provided
by three stages of regulators. The first
regulator drops the pressure from 5500 Psi to
Figure 5 - View of Graphical Interface
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500 Psi. Second stage regulator brings the
pressure from 500 psi to 50 psi, and finally from
50 to 1 psi. On the exhaust side of the fuel cell,
an on-off purge valve is used to maintain the
proper humidity, pressure and stoichiometric
conditions inside the fuel cell. This is controlled
by the power management system discussed
later.
4.3 Air SupplyThe air supply for the fuel cell is provided by a
Micronel U51DX 51mm High Performance
Radial Blower. This fan is capable of a max flow
of 16.7 CFM and max pressure of 4,900 Pa. This
blower was chosen for its performance
specifications, power usage, and weight.
4.4 Power ManagementCurrently in development is the power
management controller for the fuel cell system.
This device, developed by our team, provides
control for the air and fuel utilization by
measuring current and adjusting the air supply
blower and the hydrogen purge rate
accordingly. Also included on this board aresensors to determine the health of the fuel cell
while in flight, a data logger to record these
details during the flight and a telemetry system
for sending the readings back to the ground.
Many of the features of the power
management controller were included due to
the results of a DFEMA completed by UTRC
engineers and our team.
4.5 ByproductsThe byproducts of the fuel cell system are heat,
water, hydrogen, and air. Cut into the nose of
the aircraft are vents to provide air to the
blower as well as to remove heat, and at the tail
of the aircraft we have a vent for the escaping
air, hydrogen, and water vapor.
5 Flight TestingCurrent flight testing is focused on achieving
level flight for verifying the aircrafts general
handling and stability characteristics. These
experimental results will allow for tuning theautopilot controls and power consumption
characteristics. Due to design iterations in the
landing gear configuration and battery power
consumption, these flights are scheduled for
the first two weeks in May 2010.
A minimum of two successful test flights will be
needed; the first to determine the aircrafts
characteristics and then set the controller for
optimal power and control scheme efficiencies,
and the second to operate at optimal conditions
and record data. This will be used to perform
accurate lab tests on the fuel cell system before
installation of the fuel cell in the aircraft.
6 Future Proposed WorkWhile we are currently working towards
achieving the fuel cell long endurance flight,
there are possibilities for future work with this
aircraft. Gaseous hydrogen systems have aslightly higher specific power than existing
boro-hydride systems [1], however cryogenic
systems have roughly 10 times the power
density. We are currently investigating
possibilities of creating an insulated tank system
for use with cryogenic hydrogen, and have
spoken with some tank and specialized
materials manufacturers about such an
endeavor. Depending on funding developed
and interest from future students and external
parties, more testing will be possible to
investigate different power schemes, and flight
envelope limits.
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7 AcknowledgementsThe team has greatly enjoyed working on this
cutting edge project, gaining invaluable skills in
a variety of engineering tasks, and providing a
useful segway into graduate school and career
work. Many thanks are due to Dr. Thomas
Bradley, the pilot, Rich Schoonover, the team at
United Technologies Center, and Dr. Azer Yalin,
with the CSU Space Grant Program.
References
1. Bradley, T.H., Moffitt, B.A., Fuller, T.F., Mavris, D.N., Parekh, D.E. "Comparison of DesignMethods for Fuel-Cell-Powered Unmanned Aerial Vehicles," Journal of Aircraft, Volume 46,
Number 6, 2009.
2. Bradley, T.H., Moffitt, B., Mavris, D., and Parekh, D.E., Development and ExperimentalCharacterization of a Fuel Cell Powered Aircraft,Journal of Power Sources, Vol. 171, 2007, pp.
793-801.
3. Bradley, T.H., Moffitt, B.A., Mavris, D.N., Fuller, T.F., Parekh, D.E. "Hardware-in-the-Loop Testingof a Fuel Cell Aircraft Powerplant," Journal of Propulsion and Power 2009, Vol 25, No 6. 2009
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